Oximetry is the measurement of the oxygen status of blood. Early detection of low blood oxygen is critical in the medical field, for example in critical care and surgical applications, because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial blood, an indicator of oxygen supply. A pulse oximetry system consists of a sensor attached to a patient, a monitor, and a cable connecting the sensor and monitor.
Conventionally, a pulse oximetry sensor has both red and infrared LED emitters and a photodiode detector. The sensor is typically attached to an adult patient""s finger or an infant patient""s foot. For a finger, the sensor is configured so that the emitters project light through the fingernail and into the blood vessels and capillaries underneath. The photodiode is positioned at the finger tip opposite the fingernail so as to detect the LED emitted light as it emerges from the finger tissues.
The pulse oximetry monitor determines oxygen saturation by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor. The monitor alternately activates the sensor LED emitters and reads the resulting current generated by the photodiode detector. This current is proportional to the intensity of the detected light. A ratio of detected red and infrared intensities is calculated by the monitor, and an arterial oxygen saturation value is empirically determined based on the ratio obtained. The monitor contains circuitry for controlling the sensor, processing sensor signals and displaying a patient""s oxygen saturation, heart rate and plethysmographic waveform. A pulse oximetry monitor is described in U.S. Pat. No. 5,632,272 assigned to the assignee of the present invention.
The patient cable provides conductors between a first connector at one end, which mates to the sensor, and a second connector at the other end which mates to the monitor. The conductors relay the drive currents from the monitor to the sensor emitters and the photodiode detector signals from the sensor to the monitor.
A drawback to conventional pulse oximetry systems is the lack of standardization of the sensor and the monitor. Unless the sensor and the monitor are manufactured by the same company, it is unlikely that these two components can be connected as a functioning pulse oximetry system. This incompatibility is mainly due to physical configuration and signal parameter differences among both the sensors and the monitors. Sensors differ primarily with respect to the configuration, drive requirements and wavelength of the LEDs. Sensors also differ in the configuration and value of coding and calibration resistors used to identify, for example, sensor type or LED wavelength. Monitors differ primarily with respect to the configuration and current limit of the LED driver; the amount of preamplifier gain applied to the photodiode detector signal; and the method of reading and interpreting sensor coding and calibration resistors. Further, the physical interface between sensors and monitors, such as connector types and pinouts, is also variable. Sensor and monitor variations among various pulse oximetry systems are discussed in detail below with respect to FIGS. 1 through 3.
FIG. 1 depicts one type of sensor 100 and a corresponding monitor 150 for one type of pulse oximetry system. For this particular sensor 100, the red LED 110 and infrared LED 120 are connected back-to-back and in parallel. That is, the anode 112 of the red LED 110 is connected to the cathode 124 of the infrared LED 120 and the anode 122 of the infrared LED 120 is connected to the cathode 114 of the red LED 110. Also for this sensor 100, the photodiode detector 130 is configured so that the photodiode leads 102, 104 are not in common with either of the LED leads 106, 108.
As shown in FIG. 1, the sensor 100 is also configured with a coding resistor 140 in parallel with the LEDs 110, 120. The coding resistor 140 is provided as an indicator that can be read by the monitor 150, as described in pending U.S. patent application Ser. No. 08/478,493, filed Jun. 7, 1995 and assigned to the assignee of the present application. The resistor 140 is used, for example, to indicate the type of sensor 100. In other words, the value of the coding resistor 140 can be selected to indicate that the sensor 100 is an adult probe, a pediatric probe, a neonatal probe, a disposable probe or a reusable probe. The coding resistor 140 is also utilized for security purposes. In other words, the value of the coding resistor 140 is used to indicate that the sensor 100 is from an authorized sensor supplier. This permits control over safety and performance concerns which arise with unauthorized sensors. In addition, the coding resistor 140 is used to indicate physical characteristics of the sensor 100, such as the wavelengths of the LEDs 110, 120.
Also shown in FIG. 1 is a portion of a monitor 150 that is compatible with the sensor described above. The monitor 150 has drive circuitry that includes a pair of current drivers 162, 164 and a switching circuit 170. The monitor 150 also has a signal conditioner, which includes an input buffer 195 that conditions the output of the sensor photodiode 130. In addition, the monitor has a low-voltage source 192 and corresponding reference resistor 194 that read the sensor coding resistor 140.
Each current driver 162, 164 provides one of the LEDs 110, 120 with a predetermined activation current as controlled by the switching circuit 170. The switching circuit 170, functionally, is a double-pole, triple throw (2P3T) switch. A first switch 172 connects to a first LED lead 106 and a second switch 174 connects to a second LED lead 108. The first switch 172 has a first position 181 connected to the red LED driver 162; a second position 182 connected to a reference resistor 194 and a buffer 195; and a third position 183 connected to ground 168. The second switch 174 has a first position 181 connected to ground 168; a second position 182 connected to a low-voltage source 192; and a third position 183 connected to the infrared LED driver 164.
During a particular time interval, the switching circuit 170 causes the first switch 172 to connect the red LED driver 162 to the red LED anode 112 and simultaneously causes the second switch 174 to connect the ground 168 to the red LED cathode 114. As a result, a forward current is established in the red LED 110, which is activated to emit light. During another particular time interval, the switching circuit 170 causes the first switch 172 to connect the ground 168 to the infrared LED cathode 124 and simultaneously causes the second switch 174 to connect the infrared LED driver 164 to the infrared LED anode 122. As a result, a forward current is established in the infrared LED, which is activated to emit light. This cycle is repeated to cause the sensor to alternately emit red and infrared light. These alternating light pulses result in currents in the photodiode detector 130, which are input to a monitor buffer 166 and multiplexed 197 into an analog-to-digital converter (ADC) 199. The digitized outputs from the ADC 199, representing detected intensities, are then processed by the monitor 150 and displayed as oxygen status.
During a monitor initialization interval, the switching circuit 170 causes the first and second switches 172, 174 to be in a second position 182. This isolates the LED leads 106, 108 from the drivers 162, 164 and ground 168. Further, the low-voltage source 192 is connected to one LED lead 108 and the reference resistor 194 is connected to the other LED lead 106. As a result, a voltage is established across the parallel combination of the coding resistor 140 and the LEDs 110, 120. If this voltage is less than the forward voltage of the forward biased infrared LED 120, then, because the red LED 110 is reverse biased, neither LED 110, 120 conducts significant current. In such a scenario, the current that passes through the parallel combination of the red LED 110, infrared LED 120, and coding resistor 140 is approximately equal to the current through the coding resistor 140. Thus, the equivalent circuit is the low-voltage source 192 across the series combination of the coding resistor 140 and the reference resistor 194. The resistance of the coding resistor 140 is then easily determined via Ohms Law from the voltage across the reference resistor 194, which is read as a digitized value from the ADC 154.
FIG. 2 depicts another type of sensor 200 and corresponding monitor 250 for a conventional pulse oximetry system. This pulse oximetry system is described in U.S. Pat. No. 4,621,643 to New Jr. et al., issued Nov. 11, 1986. The sensor 200 of FIG. 2 is similar to that of FIG. 1 in that it comprises a red LED 210 and an infrared LED 220. However, in this sensor 200, the LEDs 210, 220 are in a common cathode, three-wire configuration. That is, the cathode 214 of the red LED 210 is connected to the cathode 224 of the infrared LED 220 and a common input lead 208. Also, the anode 212 of the red LED 210 and the anode 222 of the infrared LED 220 have separate input leads 202, 204. The photodiode detector 230 shown in FIG. 2 functions in much the same way as the detector 130 shown in FIG. 1 but shares one input lead 208 with the sensor LEDs 210, 220. As shown in FIG. 2, the sensor 200 also has a calibration resistor 240 with one separate input lead 206 and one lead 208 in common with the LEDs 210, 220 and photodiode 230. This resistor 240 is encoded to correspond to the measured wavelength combination of the red LED 210 and infrared LED 220.
Also shown in FIG. 2 is a portion of a monitor 250 that is compatible with the depicted sensor 200. The monitor 250 has LED drive circuitry 260 which activates the LEDs 210, 220 one at a time with a predetermined drive current independently applied to each of the LED anodes 212, 222. The monitor 250 also has a signal conditioner, including amplification and filtration circuitry 270 that conditions the input current from the detector 230, which is multiplexed 282 into a successive-approximation analog-to-digital converter (ADC) 284 comprising a comparator 285 and digital-to-analog converter (DAC) 286. A microprocessor 288 then reads the digitized detector signal for analysis. The monitor 250 reads the calibration resistor 240 by passing a predetermined current from a current source 290 through the resistor 240. The microprocessor 288 reads the resulting voltage across the resistor 240, which is passed through the multiplexer 282 and ADC 284. The microprocessor 288 then computes the resistor value per Ohm""s Law.
FIG. 3 illustrates yet another type of sensor 300 and corresponding monitor 350. This configuration is similar to those of FIGS. 1 and 2 in that the sensor 300 has a red LED 310, an infrared LED 320 and a photodiode detector 330. The configuration of the LEDs 310, 320 and the corresponding LED driver 360, however, differ from those previously described. The LED driver 360 has a voltage source 362, a red LED current sink 364 and an infrared LED current sink 367. The LEDs 310, 320 are arranged in a three-wire, common-anode configuration. That is, the red LED anode 312 and the infrared LED anode 322 have a common anode lead 302, the red LED cathode 314 has one separate lead 304 and the infrared LED cathode 324 has another separate lead 305. The voltage source output 352 connects to the common anode lead 302, the red LED current sink input 354 connects to the red LED cathode lead 304, and the infrared LED current sink input 355 connects to the infrared LED cathode lead 305.
The current sinks 364, 367 control the drive current through each LED 310, 320. The voltage source 362 has sufficient output capability to supply this drive current to each LED 310, 320 individually. Each current sink 364, 367 is a grounded emitter transistor 365, 368 having a bias resistor 366, 369 and a base control input 372, 374 that switches each transistor 365, 368 on and off. The bias resistor value and voltage of the base control input determine the amount of LED drive current. In operation, the red and infrared LEDs 310, 320 are alternately activated by pulsed control signals alternately applied to the base control inputs 372, 374.
The detector portion of the sensor 300 of FIG. 3 also differs from those in the previously minature described sensors in that a gain resistor 340 is connected to the photodiode 330. When connected to the corresponding monitor 350, the gain resistor 340 provides feedback, which adjusts the gain of a monitor preamplifier within the signal conditioner portion 380 of the monitor 350, which reduces the preamplifier dynamic range requirements. For example, if the sensor 300 is configured for neo-natal patients, where the sensor site is of relatively narrow thickness and the skin relatively transparent, the gain can be correspondingly low. However, if the sensor 300 is configured for adult patients, with a relatively thick and opaque sensor site, such as a finger, the gain can be correspondingly higher to compensate for lower detected intensities.
FIGS. 1 through 3 are examples of just some of the functional variations between sensors and monitors in pulse oximetry systems. These functional variations thwart the use of different sensors on different monitors. There are other sensor and monitor variations not described above. For example, a sensor may have LEDs with a three-wire common-anode configuration, as depicted in FIG. 7 below. There are also other potential mismatches between sensors and monitors. For example, the LED drive current supplied by a particular monitor may be either too high or too low for the LEDs on an incompatible sensor.
Besides the functional variations described above, physical variations between sensors and monitors may prevent interconnection to form a pulse oximetry system. For example, sensors have a variety of connectors. These connectors may vary from subminiature D-type connectors to flex-circuit edge connectors to name a few. Similar connector variations exist on the monitor. Further, some pulse oximetry systems require a separate patient cable, which mates to the sensor at one end and the monitor at the other end to span the distance between patient and monitor. In other systems, the sensor incorporates a cable that plugs directly into a monitor. Another physical variation is the pinouts at both the sensor connector and monitor connector. That is, there are potential differences between what signals are assigned to what connector pins.
A conventional adapter cable can sometimes be used to interconnect two dissimilar devices. The connector at one end of the adapter cable is configured to mate with one device and the connector at the other end of the cable is configured to mate with the second device. The cable wires can be cross-connected as necessary to account for pinout differences. A conventional adapter cable, however, is of little use in interconnecting various sensors to various pulse oximetry monitors. As described above, although the sensors have similar components that perform similar functions, the incompatibilities are more than connector and pinout related. In particular, a conventional adapter cable is incapable of correcting for the signal mismatches between sensors and monitors.
Although it is perhaps possible to design sensors that accommodate a variety of monitors, such sensors would be, for the most part, commercially impractical. For one, pulse oximetry sensors can be either reusable or disposable. In the case of disposable sensors, cost per sensor is critical. Even for reusable sensors, cost and complexity are important design factors. A universal sensor having integrated adapter components could be significantly more expensive than the sensors described in FIGS. 1 through 3. A sensor adapter according to the present invention solves many of the problems associated with both sensor and monitor compatibility and the need to avoid sensor complexity.
One aspect of the present invention is an adapter that provides an interconnection between a pulse oximetry sensor and a monitor. The sensor has a light source and a light detector, and the monitor has a driver and a signal conditioner. The adapter comprises a plurality of signal paths. The signal paths are detachably connected to either the monitor, the sensor or both. A first signal path is in communication with the driver and the light source. A second signal path is in communication with the light detector and the signal conditioner. The adapter also comprises an adapter element that is connected to at least one of the signal paths. The adapter element modifies a characteristic of at least one of the signal paths so that the sensor and the monitor are jointly operable to measure oxygen status. In one embodiment, where the monitor has an information element detector in communication with at least one of the signal paths, the adapter element conveys information about the sensor that is compatible with the information element detector. In another embodiment, the adapter element is connected to the first signal path and matches the light source configuration with the driver configuration. In yet another embodiment, the adapter element is connected to the first signal path and matches the drive requirements of the light source with the drive capabilities of the driver. In an additional embodiment, the adapter element is connected to the second signal path and provides gain for a detector signal.
Another aspect of the present invention is a sensor adapter comprising a sensor having a light source and a light detector and comprising a plurality of signal paths. The signal paths are detachably connected to a monitor. A first signal path communicates a drive signal from the monitor to the light source. A second signal path communicates an intensity signal from the light detector to the monitor. The sensor adapter also comprises an adapter element in communication with at least one of the signal paths. The adapter element creates a compatibility signal that allows the sensor and the monitor to be jointly operable as a pulse oximetry system. In one embodiment, the sensor adapter comprises an active component. The active component generates a predetermined signal level applied to the first signal path that conveys information regarding a compatible sensor. In another embodiment of the sensor adapter, the light source has a conductive portion with a predetermined equivalent resistance that conveys information regarding a compatible sensor. Advantageously, the conductive portion may be an LED encapsulant or incorporated within the semiconductor material of an LED. In yet another embodiment, the sensor adapter further comprises a translator that senses a sensor information element and communicates equivalent information to the monitor.
Yet another aspect of the present invention is a method of connecting an incompatible sensor to a monitor. The method comprises the step of adapting a signal in communication with either the sensor, the monitor or both so that the sensor and the monitor are jointly operable as a pulse oximetry system. In one embodiment, the adapting step comprises the steps of sensing a drive signal and switching the drive signal to a particular one of a plurality of light source leads in response to the drive signal. Advantageously, the switching step may connect a two-wire driver to a three-wire light source or may connect a three-wire driver to a two-wire light source, either connection being made through a multiple-pole, multiple-throw switch. In another embodiment, the adapting step comprises adjusting a drive signal from the monitor to match the drive requirements of a light source in the sensor. In yet another embodiment, the adapting step comprises providing a feedback signal to the monitor. The amount of the feedback determines the gain applied within the monitor to a light detector signal from the sensor. In an additional embodiment, the adapting step comprises generating an information signal to an information element detector that corresponds to information from a compatible sensor. In another embodiment, the adapting step comprises translating an information signal from a sensor into a translated information signal that is read by an information element detector and corresponds to a compatible sensor.
A further aspect of the present invention is a sensor adapter for operably interconnecting an incompatible sensor to a monitor in a pulse oximetry system comprising an interconnect means for providing a signal path between the sensor and the monitor. The sensor adapter also comprises an adapter means for creating a compatible signal on the signal path. In one embodiment, the adapter means comprises a configuration means for routing a drive signal from the monitor so as to correspond to a light source in the sensor. In another embodiment, the adapter means comprises a limit means for changing the amount of a drive signal from the monitor so as to correspond to a light source in the sensor. In yet another embodiment, the adapter means comprises a gain means for modifying the amplitude of a detector signal from the sensor. In an additional embodiment, the adapter means comprises an information means for providing a signal to an information element detector that corresponds to a compatible sensor.